Confusing thought on APS-C Vs FF Vs any other format you choose

I am currently looking for a replacemant for my ageing 300D as it has now suffered the AF Sub Mirror failure. My prefered subject matter is aviation and a bit of adventure sport (My son is training to be an adventure sports instructor). I have been shooting aviation since by early teens (late 70's) starting with my farthers old Pracktica Nova B with a 135mm Carl Zeiss Jena lens with 2× converter. I then moved on to a Pentax ME Super eventually shooting with a 80-200mm again with 2× converter. Back then I shot with BW and E6 Slide (did all my own D&P as well, happy days in the dark!).
Fast forward to now, and I have been using my 300D with Sigma 28-300, and have fallen into what I think is a bit of a trap. My 300mm lens now it is on my APS-C sensor has the FoV of a 480mm on the old FF. So although I only have 300mm it is still better than when I shot 400mm on FF.
I think that actually this is not true, If I am shooting a fixed sized object, i.e the aircraft, at a fixed distance. Most airshows have the flight line at about the same distance from the crowd line so the distances stay reasonably close from show to show. Now the realisation that I had was that a 300mm lens will reproduce the fixed size × distance image at the same size on the sensor irrespective of the actual sensor size, be it APS-C or a 10"×8" plate. Until the image formed on the sensor actually fills that sensor then the format "FoV" in relation to the focal length of the lens actually makes no difference to the actual size of the image recorded. So if I am still having to crop on any format then I have gained nothing in "Effective focal lenth". In the old days when shooting on film then the size of the finnished image that could be produced was dependant on the quality of the film that you were using (slow speed fine grain / high speed lots of grain) and of course the quality of your enlarger lens! Assuming that one was using the best (or at least equivalent quality) camera lenses avilable. I know that pixels don't really have a "size" but again we all accept that there are normally accepted pixel resolutions for printing, so for example if we are printing for an image that is likely to be held in the hand (or viewd form an equivalent distance) then 300 ppi is accepted as being an optimum resolution for example. Now unfortunatly even with unlimited money for some of the new super Tele's or Tele zooms such as Sigmas 300-800mm (which would be my focal range of choice now) are so big and heavy that they are only useable from a tripod, which is something I have never been able to use for aviation. Looking back at my images 600mm seems to offer a reasonable reach, well at least being almost able to almost fill the APS-C frame when the ship is level with you along the flight line, although I find that my current 500mm is a bit short a lot of the time especially when the ship is pulling up at the ends of runs.
So even with a lens in the 500-600mm range needing croping on both the APS-C and FF sensors (I know that the FF crop is going to be 60% more). So the total number of pixels is not really of much interest, but the pixel density is, as what we are limited by is the essentially fixed size of the image on the actual sensor.
Now I hope that at least some of you are still with me. Am I correct in the statements that I have made above, and that what I need is a good pixel density within the limits of IQ? This is different from what the majority of users are looking for when they are in a position to fill what ever size frame they have with the image that they want.

Comments on my thoughts are very welcome.

Alan

Yes, you are right. When the subject doesn't fill the frame, and you will need to crop some away regardless of format size, then crop factor is irrelevant. What gives more 'reach' is pixel density. More pixels on target will give you a larger image of that subject.

Thanks for confirming my thoughts. I guess that now I need to look at pixel densities, as well as all the other features in a body. The other thing is I hope that Sigma can get the 120-300 f2.8 out into the shops, with the 2× converter, at least then I can have 240-600mm with AF, in a package that is potentially had holdable. I have seen the 300-800 which focal length wise would be my perfect choice, trouble was I just couldn't lift it from the ground, let alone attach a body to it and use it! Given the size of that lens you hardly say you fit it to a body. BTW dose the Canon 100-400 AF with a 2× converter?

Alan

BigAl007 wrote in post #11600131
BTW dose the Canon 100-400 AF with a 2× converter?

Alan

Nope, it won't AF on any camera with a 2xTC and you would need a 1D to get it to AF even with just a 1.4xTC on. So any APS-C body will need manual focusing with either converter. You can try taping the pins, or using a non-canon brand TC that doesn't report, but the AF will be compromised anyway, very slow and liable to hunt.

The 100-400L doesn't really like TCs much either, I find it gives decent results with a 1.4x but images with the 2x are starting to show the quality drop.

You have less 'cropability' when using a converter as the IQ drop starts to become more noticeable as you crop. It is debatable as to whether the quality loss with the 1.4x is better or worse than using the lens without converter and cropping in more.

I personally don't use the TCs on the 100-400L after initial trials, but still have them as they work brilliantly on the 300 f/2.8L and so that gives me 420 f/4 and a 600 f/5.6 options. It is also possible to stack them to get an 840 f/8 but that loses me the AF again.

Perhaps it's time to look at a non-TC'd long lens, like http://www.usa.canon.c​om …_pro/ef_600mm_f​_4l_is_usm

A big prime would be nice but although I twittered on about the size of the object being imaged being constant, they do actually come in different sizes, most of the military combat types are roughly ythe same size, but there are some smaller civilian types to be seen, along with some of the big transport types. This makes a long focal length zoom very useful. I think that actually two bodies with two lenses could cover everything. The first would be the 28-300mm zoom, which is perfect for formations, and large transport types. I haven't seen too many comments on the 28-300 L glass, I know that it is a trombone type zoom, which is not popular with many, but not sure about what the optical quality is like. I have been using the Sigma 28-300mm and the focal length range is very good.
The other lens that would be perfect is the Sigma 300-800 f5.6. This is IMO just about the perfect focal length range for air shows. There is though one very large problem with this lens, it's very very very large (about 3 foot long and a foot in diameter), and very very very very heavy! When I had a look at one in a shop I could not actually lift the lens, let alone use it. I saw a 120-300 f2.8 yesterday, and it is physically at the top end for useablity but is hand holdable.

Alan